This invention relates to semiconductor vertical cavity surface emitting lasers (VCSELs), and more particularly to structures and techniques for providing highly-efficient and single mode VCSELs. A VCSEL is a semiconductor laser consisting of a semiconductor layer of optically active material, such as gallium arsenide or indium gallium arsenide or the like, sandwiched between highly-reflective layers of metallic material, dielectric material, epitaxially-grown semiconductor dielectric material or combinations thereof, most frequently in stacks. As is conventional, one of the mirror stacks is partially reflective so as to pass a portion of the coherent light built up in the resonating cavity formed by the mirror stack/active layer sandwich. Laser structures require optical confinement and carrier confinement to achieve efficient conversion of pumping electrons to stimulated photons (A semiconductor may lase if it achieves population inversion in the energy bands of the active material.) The standing wave in the cavity has a characteristic cross-section giving rise to an electromagnetic mode. A desirable electromagnetic mode is the single fundamental mode, for example, the HE.sub.11 mode of a cylindrical waveguide. A single mode signal from a VCSEL is easy to couple into an optical fiber, has low divergence and is inherently single frequency in operation.
VCSELs are known in several forms based on the location of the active layer relative to the mirror stacks and the location of the electrodes. Previously-known categories are index-guiding or gain-guiding of the optical mode, and either vertical injection (vertically along the central axis of the cavity) or lateral (radial) injection of carriers into the active region. As used herein, vertical and axial are used interchangeably and lateral and radial are used interchangeably.
Reference is made to a survey article describing the various categories entitled "High-efficiency Vertical Cavity Lasers and Modulators," by Coldren et al., SPIE Proceedings Vol. 1362 (Physical Concepts of Materials for Novel Optoelectronic Device Applications II: Device Physics and Applications, presented 28 Oct-2 Nov 1990, Aachen, Germany, Copyright 1991).
The previously-known VCSELs suffered from relatively-low power efficiencies, which can be related to geometry limitations or low pumping efficiency. Their small size and power inefficiency caused the devices to become hot as the input power is increased. Since the lasers tended to self-quench at higher temperatures, the light output power has been limited.
At high currents, vertically-injected lasers tend to overheat because of the inherent series resistance of the mirror structures, while laterally-injected lasers tend to develop non-uniform current distribution with "current crowding" at the edges of the active region as the injection current increases. As the optical mode is weak at the edges of the active region, conversion efficiency is reduced substantially. Therefore, laterally-injected structures typically have been limited to significantly lower saturated power output levels than vertically-injected lasers, due to the supposed inherent structural limitations. Moreover, due to mismatch between known injected current patterns and the desirable lateral optical modes of VCSELs, injection current patterns produced by prior art devices result in undesirable multimode behavior at elevated power levels.
A further category of VCSELs has been developed by a research group based at the University of California at Santa Barbara under the direction of one of the co-inventors, Professor Larry A. Coldren. Referring to FIG. 1, a lateral-contact VCSEL 10 was developed which has a first electrode 12 disposed as a ring or toroid around the base of a first mirror stack 14, which serves as an index waveguide, a second electrode 16 being disposed on the back of the semiconductor substrate 18 such that the active region 20, which is formed of a conventional GaAs multiple quantum well, is contacted on opposing sides by the electrodes 12 and 16 and sandwiched between first and second optically transmissive contacting layers 22, 24 abutting respectively the first mirror stack 14 and a second mirror stack 26.
The suggestion has been made in a public forum in the United States (UCSB Electrical and Computer Engineering Research Review, Santa Barbara, Calif., January 1991 and the Conference on Lasers and Electro-Optics, Anaheim, Calif., May 1991), in the course of development of the present invention, that a current confinement aperture 27 be provided around the active region to funnel current into the active region directly under the silhouette of the first mirror stack 14. The proposed current confinement aperture was to be formed by wet etching, and thus the concurrent large optical index discontinuity must be kept outside the cavity to avoid disturbing the electromagnetic mode. It is believed that a similar structure has since been incorporated into devices developed by others, such as a group headed by Jack Jewell and Greg Olbright at Photonics Research in Colorado. Further, at these public forums, it was suggested that a linear vertical grading be provided in the top or first contacting layer 22 as one technique to minimize current crowding. An attempt was made to fabricate such a device. However, deficiencies were discovered, and further research resulted in the present invention.
What is needed is a VCSEL with a structure which overcomes the inherent current crowding inefficiencies of lateral electrode configurations and the inherent power conversion inefficiencies of vertical electrode configurations.